The present invention relates to improvements in pilot-operated sliding-spool directional control valves which preferably serve to regulate the flow of a hydraulic fluid to and from a consumer, e.g., to and from double-acting actuator cylinders. More particularly, the invention relates to improvements in sliding-spool directional control valves which are preferably operated by electromagnetic pilot valve means.
It is known to provide the body of a sliding-spool directional control valve with a pressure port which is connected to a pump or another suitable source of pressurized fluid, with two working ports which are connected to a consumer, and with two return ports which are connected to a reservoir. The spool of such a directional control valve has a center piston or land which is disposed between the working ports and can seal one working port from the pressure port while the pressure port communicates with the other working port or vice versa, a first outer piston or land which seals the one working port from the associated return port when the one working port communicates with the pressure port, and a second outer piston or land which seals the other working port from the associated return port when the other working port communicates with the pressure port. All three lands are rigid with a spindle or shank of the valve spool. Therefore, the center land must invariably engage one of two centrally located seats while one of the outer lands sealingly engages an adjacent seat in the valve body, and the center land must invariably engage the other of the two centrally located seats while the other outer land sealingly engages an adjacent seat in the valve body. Such directional control valves are not reliable because the sealing action is often unsatisfactory due to manufacturing tolerances and/or as a result of wear upon the lands and/or seats.
Attempts to reduce the likelihood of leakage as a result of manufacturing tolerances and/or wear include the provision of a spool wherein one of the lands is movable lengthwise of the shank. Such directional control valves are quite complex and expensive. Moreover, the number of component parts is very high and the sealing action is still not entirely satisfactory, especially as concerns the retention of lands in sealing engagement with the respective seats.